1. Field of the Invention
The present invention relates to an electric compressor which compresses gas with the rotation of an impeller, by rotating a rotary shaft and the impeller with a motor unit.
2. Description of the Related Art
In the conventional electric compressor, for example, there is disclosed a turbo machine equipped with vane wheels (hereinafter referred to as impellers) to both sides of the rotary shaft, in Japanese Patent Laid-Open No. H11-13686 (hereinafter referred to as Patent Document 1). Generally, in this type of compressor, a thrust load proportional to a square of a number of rotations of the impeller generates in the axial direction of the rotary shaft, when the number of rotations of the impeller increases. In the turbo machine disclosed in Patent Document 1, the thrust load is cancelled by making the thrust load generated by the impellers provided to both sides of the rotary shaft to act in a direction opposite to each other. Therefore, in this turbo machine, a dynamic gas bearing, a so-called thrust air bearing, is adopted as a thrust bearing which positions the rotary shaft in the axial direction.
Recently, an attempt for mounting a fuel cell which generates electricity using hydrogen and air (oxygen) to a vehicle is entering a practical phase. In the fuel cell system, there are types where power generation is carried out by supplying compressed hydrogen to the anode side and compressed air to the cathode side. In these types of fuel cell system, a compressor for compressing air is mounted to the vehicle. Such compressor is required to be compact in size and light-weight, from the requests such as downsizing and improvement in fuel consumption of the vehicle. From such viewpoint, the turbo machine disclosed in Patent Document 1 is equipped with the impellers at both sides of the rotary shaft, so that the structure of piping for supplying compressed air to the fuel cell becomes complicated, which makes downsizing and reduction in weight difficult. Further, in this type of the turbo machine, it is necessary to reduce cost to make the same practicable. However, the impeller needs to be light-weight and have high mechanical strength, therefore the cost of the material thereof is high. Further, high precision is necessary in order to process the impeller, therefore the cost of the production thereof is also high. As such, the turbo machine disclosed in Patent Document 1 requiring a pair of impeller is difficult to reduce cost.
However, when only one impeller is provided to one side of the rotary shaft in order to downsize and reduce cost of the compressor, the thrust load proportional to the square of the number of rotations generates as the number of rotations increases, so that the thrust air bearing disclosed in Patent Document 1 cannot withstand the thrust load.
On the other hand, in Japanese Patent Laid-Open No. 2006-234075 (hereinafter referred to as Patent Document 2), there is disclosed a fluid compressor capable of being used in the fuel cell system. The fluid compressor is equipped with a balance piston of a disk shape protruding in the outer peripheral direction of the rotary shaft, a balance chamber provided inside a housing for accommodating therein the balance piston, and an aperture portion provided at the gas outlet side of the balance chamber and which changes the dimension of the fluid channel with the displacement of the axial direction of the rotary shaft with respect to the housing. In the fluid compressor disclosed in Patent Document 2, when the thrust load increases with the increase in the number of rotations of the rotary shaft, the position of the rotary shaft inside the housing changes by the thrust load. In accordance thereto, the fluid passage of the aperture portion is narrowed and the pressure of gas inside the balance chamber increases. By doing so, the load bearing ability of the balance chamber and the balance piston in the thrust direction is improved, so as to make the same capable of withstanding the thrust load.
However, when the rotary shaft moves in the axial direction inside the housing, the clearance between the impeller mounted to the rotary shaft and the casing of the compressor facing the impeller changes. In this type of compressor, the clearance between the impeller and the casing exerts serious effect to the compression efficiency and the surge property. Therefore, the change in the clearance leads to decrease in the compression efficiency and degradation in the surge property. As such, it becomes difficult to supply the compressed air to the fuel cell effectively, and further there is a fear that abnormal noise or abnormal vibration may occur from surging according to circumstances.